1. Field of the Invention
The present invention relates to a method for manufacturing a light-emitting device using an evaporation method.
2. Description of the Related Art
Light-emitting elements which include organic compounds as a light-emitter and are characterized by thinness, lightweight, fast response, and direct current driving with low voltage are expected to be applied to next-generation flat panel displays. In particular, a display device in which light-emitting elements are arranged in a matrix has advantages in a wide viewing angle and excellent visibility over a conventional liquid crystal display device.
A light emission mechanism of a light-emitting element is described below. When voltage is applied between a pair of electrodes by which an EL layer is sandwiched, an exciton is formed with an electron injected from a cathode and a hole injected from an anode. When the exciton recombines at an emission center in the EL layer and relaxes to the ground state, energy is released as light from the EL layer. As excited states, a singlet excited state and a triplet excited state are known, and it is thought that light emission is possible through either of those excited states.
An EL layer included in a light-emitting element has at least a light emitting layer. The EL layer can have a stacked-layer structure including a hole-injecting layer, a hole-transporting layer, an electron-transporting layer, an electron-injecting layer, and/or the like in addition to the light emitting layer.
EL materials for forming the EL layer are broadly classified into a low molecular (a monomer) material and a high molecular (a polymer) material. In general, a film of a low molecular material is often formed by an evaporation method and a film of a high molecular material is often formed by an ink-jet method or the like.
An evaporation apparatus which is used in an evaporation method has a substrate holder to which a substrate is mounted; a crucible (or an evaporation boat) containing an EL material, that is, an evaporation material; a heater for heating the EL material in the crucible; and a shutter for preventing the EL material from being scattered. The EL material which is heated by the heater is sublimated and deposited onto the substrate.
Note that in order to achieve uniform deposition, actually, a deposition target substrate needs to be rotated and the substrate and the crucible need to be separated from each other by at least a certain distance. In addition, when films of different colors are separately formed using a plurality of EL materials through a mask such as a metal mask, it is necessary that the distance between pixels be designed to be large so that the width of a partition (a bank) formed of an insulator between pixels is large. Such needs are major problems in advancing high definition (increasing the number of pixels) and miniaturization of pixel pitches along with downsize of a light-emitting device including a light-emitting element.
Therefore, as for flat panel displays, it has been necessary to solve those problems as well as to achieve high productivity and cost reduction in order to achieve higher definition and higher reliability.
Thus, a method for forming an EL layer of a light-emitting element by laser thermal transfer has been proposed (see Patent Document 1). In Patent Document 1, a transfer substrate is described which has, over a supporting substrate, a photothermal conversion layer including a low reflective layer and a high reflective layer and a transfer layer. Irradiation of such a transfer substrate with laser light allows the transfer layer to be transferred to an element-forming substrate.
[Patent Document]
    [Patent Document 1] Japanese Published Patent Application No. 2006-309995